Regarding Kuma's actions towards the end of Thriller Bark, Luffy's health bar was in the red, barely visible. Zolo was able to take Luffy's damage, because he didn't lose a lot of health.
He was still standing, because he had some health left. Luffy couldn't stand beforehand, because he took the most damage.
The reason Luffy couldn't stand up is from his two chunks of damage he took when he 'lost' to Crocodile. Zolo probably took the damage that Luffy took because he knows that Crocodile is most likely as strong as Moria, and he didn't take much damage himself.
Nico Robin lines up with Crocodile, while Zolo lines up with Mihawk. Zolo had battled against Mihawk, and took damage from Crocodile.
It's possible for there to be a story line involving Robin, Mihawk, Zolo, Crocodile, and Luffy. Maybe Mihawk is after Uranus, and Robin might stand in his way?